Conventional digital multimeters have ranges which are related by factors of 10. For example, a representative 31/2-digit multirange DC voltmeter might have ranges of 0.000-1.999, 00.00-19.99 and 000.0-199.9 volts, with resolutions of 0.001, 0.01 and 0.1 volts per digital step respectively. Such an instrument might have a 3-position range switch in the form of a 3-position voltage attenuator connected between the voltage input terminals and a constant sensitivity analog-to-digital converter. The scales, ranges and resolutions for such a representative conventional digital voltmeter are listed in Table 1.
Digital multimeters typically use a dual slope integrator circuit to generate a pulse whose duration is proportional to an input voltage. The integrator circuit pulse duration is digitally measured by counting the number of clock pulses from a clock oscillator which occur during the integrator circuit pulse. In a typical 31/2-digit meter, the counter range will be 0000 to 1999, with the counted digital values being displayed on a 31/2-digit visible display.
With such digital multimeters, it is sometimes inconvenient that the resolution changes substantially from one range to the next. For example, with the previously described 31/2-digit meter, a measurement of a value of approximately 1.5 volts can be made on the 2 volt scale with a resolution of 0.001 volts or 0.07% of the reading. However, a measurement of a value of approximately 2.5 volts must be made on the 20 volt scale, with a resolution of 0.01 volts or 0.4% of the reading.